Covering groups and their integral models
Covering groups and their integral models
Given a reductive group $\mathbf {G}$ over a base scheme $S$, Brylinski and Deligne studied the central extensions of a reductive group $\mathbf {G}$ by $\mathbf {K}_2$, viewing both as sheaves of groups on the big Zariski site over $S$. Their work classified these extensions by three invariants, for $S$ …